How the test was made
We carry out our tests ourselves and test all products as they are intended to be used in reality. We first took the thermoses out into the wilderness, where they were used by two outdoor-loving families. The families were asked to open, pour, drink from, drop and handle the thermoses in all possible ways, with and without gloves and with both small and large hands.
We then carried out more controlled tests both indoors and out, such as filling each thermos with boiling water and measuring the temperature after 30 minutes, one, two and four hours. We also stood them upside down to see if they leaked.
In our assessment we focused on the following areas:
Heat retention capacity: How effectively does the thermos keep the contents hot?
Construction: How impact and shock resistant is the thermos? How well does it prevent leakage? How well designed is it? How heavy is it and how big is the opening you drink from?
Ease of use: How light, neat and stable is the thermos? Can you remove and replace the lid even with damp gloves or weak hands? Can thermoses with snap lids be resealed easily? Does it fit into a smaller rucksack? How easy is it to clean? Is it dishwasher safe? How clear are the instructions?
We gave each thermos a score according to its value for money; in other words, how good it is in each area in relation to its price tag. We thus have higher expectations of an expensive product than a cheaper one, and vice versa. We counted thermoses costing less than £25 as budget class, and those costing between £25 and £40 as medium class. If they cost more than this, we counted them as premium buys.
We've chosen the Thermos King Flask as our best in test. This is a true classic, made in a factory established in 1929 by the Thermos company after which all thermos flasks are named. The Thermos King Flask, with its attractive retro style and metallic paint, is made from uncrushable stainless steel and copes with our leak and drop tests without problems. The King Flask maintains high temperatures throughout our tests, both outdoors and indoors, while the outside remains cool. The lid is easy to use even for children and elderly people, and can be removed and replaced while wearing gloves. The opening could be larger to facilitate cleaning, however. The Thermos King Flask is ideal for someone who wants an attractive thermos that does what it promises, and does it well.
The Stanley Adventure Vacuum Bottle is a substantial thermos in classic style, manufactured in stainless steel. This thermos was invented way back in 1913, but the design doesn't seem to have aged and the thermos keeps warm well both during indoor and outdoor testing. In addition, it's leak proof, tolerates being dropped and is easy to manage even for younger children. The construction is simple with a screw top and a normal thermos mug. The only thing we found to be slightly negative is that the thermos opening is a bit too small, which makes cleaning more difficult. The Stanley Adventure Vacuum Bottle is quite simply a stylish, reliable classic for every trip out.
Quickstop is made by the German company Isosteel, and is a low priced and relatively light but still sturdy thermos in stainless steel with a screw cap and thermos mug. This is a flask with no frills except the leak protection that works automatically when the mug is screwed on. In other words, it's just a standard thermos. But that's saying quite a lot. The Isosteel Quickstop stays cool on the outside and has top class heat retention capacity. The only disadvantages are that the opening's a bit small, which makes it hard to clean, and that the lid is rather slippery to handle when you're wearing gloves. It copes with the leak test without problems, but the thermos ends up with two small dents when it's dropped on the ground, which reduces the overall score slightly. However, the Isosteel Quickstop is overall a good value for money budget thermos for everyday use.
The Primus Trailbreak Vacuum Mug, from Swedish company Primus, is a large, stable thermos in stainless steel, with silicon gaskets that mean the cup doesn't leak, and a generous opening. It includes two lids as well as the actual drinking cup - a normal lid and a snap lid for when you want to drink without taking the lid off. This is a nice idea, but the snap lid isn't as easy to keep clean and breaks more easily than a standard cup. It also means you have a lot of parts to keep track of. Despite the fact that the outer cup lid on the Primus has grip-friendly grooves, it's quite difficult for anyone with small hands to open. The thermos has a cool outside and is robust enough to cope with both drop testing and leak testing. The Primus Trailbreak Vacuum Mug is a good buy for anyone who wants a substantial thermos with a choice of different types of cup.
The Hydro Flask Wide Mouth Flex is a small, lightweight and easy to manage thermos made from stainless steel, the outside of which doesn't get hot. The large opening makes it easy to clean. The lid has a handy rubber handle that's easy to get a grip of, even for cold, fumbling hands after a day on the ski slopes. The Wide Mouth Flex has no problems with our drop or leak testing. However, the thermos goes cold relatively quickly, both indoors and in winter temperatures outdoors. The Hydro Flask Wide Mouth Flex is therefore most suitable for short adventures or for your morning coffee on the way to work.
Klean Kanteen is making a major investment in manufacturing water bottles and thermoses in stainless steel instead of plastic, with a view to reducing environmental impact. The lid on the Insulated Classic thermos is easy to use and the flask doesn't get hot on the outside or leak. However, it did get a dent during our drop testing. The thermos opening is rather small, which makes cleaning difficult, and the thermos doesn't keep drinks hot for very long. However, the Insulated Classic gets a plus for the innovative design and the striking colours. Klean Kanteen's Insulated Classic is above all a cool thermos that feels like it was developed for city-dwellers with a caffè latte in one hand and a laptop in the other.
The Eva Solo To Go Cup is a smaller thermos in stainless steel with a silicon snap lid, which is great if you don't want to have to unscrew a cup. However, the snap lid means a bigger risk of leaks, because if there's any remaining liquid in the lid it may leak out. Despite a useful strap in the cup, it's a little difficult for smaller children to open. The thermos is dishwasher-safe, which is unusual and is a big plus point for anyone who doesn't like washing dishes by hand. It doesn't leak when laid on its side, and nor does it break when dropped on the ground. However, the outside gets a little warm when filled up with hot liquid and the thermos doesn't stay hot for very long. The Eva Solo To Go Cup isn't something you'll want to take on a full day trip, but just as the name implies it's fine as an easy-to-manage thermos to fill with coffee and shove in your bag when you're on the go.